This invention relates to fuel pellets for use as targets in a device employing thermonuclear fusion. The invention further relates to high polymer hydrocarbons in which tritium has been substituted for bound hydrogen. And finally the invention relates to the manufacture of pellets in the microgravity environment of space.
Hollow gas-filled glass or metal spheres or target pellets are known (see Wang and Elleman, U.S. Pat. No. 4,344,787, xe2x80x9cMethod and Apparatus for Producing Gas-Filled Hollow Spheresxe2x80x9d). Molten material is forced from a nozzle by a piston while a high-pressure gas fills the center of the liquid column as it emerges from the pipe. The molten material emerges into a gas-filled environment and breaks into hollow spheres.
Nuckolls et al., U.S. Pat. No. 4,376,752, xe2x80x9cFoam Encapsulated Targetsxe2x80x9d uses a similar process to fabricate glass or metal target pellets containing thermonuclear fuel. However, vibrations are applied to the extruded tube to assist its breakup into uniform hollow bodies. Teitel et al., U.S. Pat. No. 4,432,933 discloses the use of not only glass and metal but also ceramic, carbon and plastic in manufacturing hollow microspheres for application in thermonuclear fusion reactors.
The present invention utilizes high polymer hydrocarbons in which bound hydrogen has been replaced by tritium, an isotope of hydrogen, as shell materials for fuel pellets to be used in thermonuclear fusion devices employing inertial confinement. The required high polymer can be produced from monomer(s) in which hydrogen has been replaced by tritium. Pellets fabricated from tritium substituted high polymer are filled with thermonuclear fuel.
Because the chemistry of hydrogen and tritium are similar, all plastics are capable of such substitution whereby bound hydrogen has been replaced by tritium. Thermoplastics, however, are favored in the present invention because of their relative ease of fabrication into pellets. Nevertheless there are no inherent reasons why thermosetting resins cannot be employed. The thermoplastics of greatest interest include such polymers are polyolefins, polystyrene, polyamides, polyesters, acetal copolymers, polyacrylonitrile, and aromatic polyamides/imides.
Plastics possess many properties that make them attractive for use in target pellets. These properties determine not only the success with which a given plastic can be fabricated into a fuel pellet but also the performance of the pellet so formed. Properties of a given plastic which must be considered include the following attributes: strength to weight ratio, toughness, tensile strength, optical clarity, chemical resistance, thermal stability, resistance to radiation degradation, solvent resistance, gas permeability, flammability, mass density, and products of decomposition.
One factor which is critical in selecting a material for use in fabricating target pellets is its average atomic number, or Z number.
Theoretical and experimental results indicate that materials with low Z numbers are preferred. This consideration makes plastics more attractive than other materials. The lower effective atomic number of plastics provides a relatively more efficient utilization of the incident radiation and therefore promises greater energy gain, defined as the ratio of the fusion energy produced to the input radiation energy (commonly laser beams).
The conscious substitution of tritium for hydrogen in plastic used for target pellets is based on a consideration of its nuclear properties. First, both tritium and hydrogen have the same Z number so that such a substitution will not affect the average atomic number of the plastic. Second, tritium is a component of deuterium-tritium (D-T) mixtures, the most promising fuel for thermonuclear fusion. By contrast, hydrogen is a poor fuel for fusion, requiring, by orders of magnitude, greater temperatures to ignite. Third, compared with hydrogen, the tritium nucleus has very close to triple the mass. And fourth, whereas tritium does not absorb neutrons, hydrogen possess a neutron cross section of 0.33 barns.
To understand the significance of the differences in nuclear properties between tritium and hydrogen, one must consider the mechanism of an implosion. When fuel pellets fabricated from a conventional plastic are subjected to intense radiation such as laser beams, the plastic instantly becomes a plasma. All chemical bonds are broken and electrons are disassociated from atomic nuclei. Because the hydrogen nuclei (protons) are much lighter than the carbon nuclei (or other nuclei), the former will diffuse more rapidly into the D-T fuel mixture and thereby compress it. In order to achieve the most efficient compression, they front of the imploding plasma needs to be as symmetrical as possible.
Any effects which reduce the symmetry of the imploding front have been demonstrated to reduce the compression of the fuel. In an experiment carried out at Lawrence Livermore National Laboratory, an applied 15 percent asymmetry to produce a pancake configuration of the fuel resulted in a 40-fold drop of the neutron yield (E and TR, July-August, 1988, p. 34). Thus, measures are taken to fabricate targets as symmetrical as possible and to subject the targets to laser beams arranged in a circumferentially uniform pattern.
Another potential source of asymmetry comprises the instabilities set up along the imploding plasma front. As an example, Raleigh-Taylor instabilities may be formed at boundaries between materials of different masses. If a heavy material is accelerated against a light material, the boundary between the two will be stable. But if a light material is accelerated against a heavy material, the boundary between the two will be unstable and turbulent, causing the two materials to mix in a way extremely difficult to predict. Therefore, one can expect instabilities to result when protons are accelerated against a D-T mixture, but substitution of tritium nuclei for protons would mitigate any such effect.
Whatever tritium might become commingled with the D-T fuel would have far less effect than protons. The latter, as pointed out, is a poor fuel for fusion and therefore would have the effect of diluting the reactants. Tritium, on the other hand, can react with the fuel as if it were part of the original fuel mixture. At most, a compensation in the ratio of the D-T fuel components might be needed.
Once ignited, the D-T fuel gives off a profusion of neutrons as fusion occurs. These neutrons are not required in the reaction, and, if anything, serve only to dilute the reactants and slow the reaction. Thus, the neutrons should be allowed to escape from the reaction zone. Pellet materials with low cross sections such as carbon will least impede the outward diffusion of neutrons. Likewise, neutrons should interfere less with an imploding front of tritium than with protons.
Because tritium is radioactive, with a half life of 12.3 years, it will slowly decay. As a result, any plastic containing bound tritium will gradually disintegrate. Obviously, this effect is undesirable, but it is not as critical as it might seem. Resins whose skeletons contain carbon-carbon linkages are less prone to rupture than those containing carbon-oxygen or other bonds. Therefore, hydrocarbon polymers would be preferred. In this case, tritium decay would lead primarily to the formation of free radicals and cross-linking between polymer chains.
Hydrocarbon polymers are preferred for another reason as well as their radiation resistance. Having a lower average Z number they promise greater energy gain. First consideration therefore is given to such volume-produced hydrocarbon resins as polyethylene, polypropylene and polystyrene. Tritium can be substituted for hydrogen in these plastics by using organic syntheses to prepare the corresponding monomers starting with tritium oxide as a source of tritium. Monomers can also be prepared by isotope exchange. The substituted polyolefins have an advantage over substituted polystyrene in that the ratio of T/C is 2:1 instead of 1:1 thus providing a lower average Z number. On the other hand, polystyrene, because of its aromatic structure, has been shown to be more radiation resistant.
The hydrocarbon resins are soluble in common solvents and therefore can be processed by solvent casting techniques. Conventional processes may be applied to the fabrication of fuel pellets, but to achieve superior results, manufacture in space or near zero gravity is recommended. In this manner, pellets will be provided with improved sphericity and concentricity, both of which are essential to their use. A symmetrical shell will not only create a higher energy gain but can also withstand higher gas pressures. Manufacture in space will also produce greater uniformity of pellets so that fewer rejects will be produced.
In summary, fuel pellets are made in a process that begins by producing monomers containing exclusively carbon and tritium. These monomers can be prepared by chemical synthesis. Alternatively, such monomers can be prepared by iosotope exchange whereby tritium is substituted for hydrogen in the corresponding monomer. This reaction is known as the Wilzbach exchange.
Monomers that can be made by such means include tritium substituted ethylene, propylene, butene, butadiene, styrene and pentadiene. These compounds as well as other hydrocarbon monomers are polymerized by methods well known in the art. Either homopolymers or copolymers can be produced.
The resulting high polymer hydrocarbon, containing only carbon and tritium, is formed into rigid, hollow pellets that are substantially spherical in shape and have walls of uniform thickness and density. This means that the wall of each pellet is a continuous solid phase of high polymer hydrocarbon.
Pellets, once formed, can be treated by means known to the art to cross link the polymer chains. Improved physical properties may be obtained by such cross linking. Prior to use, pellets are filled with thermonuclear fuel. When blowing microballoons it is possible to introduce the fuel during the fabrication step, but the preferred way is to fill the pellets after they have been formed.
It is therefore an object of the present invention to produce high energy gain fuel pellets for use in thermonuclear fusion reactors employing inertial confinement at sufficiently low cost to allow their commercial exploitation.
Another object of the invention is to produce fuel pellets having uniform shell thickness and density.
Still another object of the invention is to produce fuel pellets that are spherically symmetric so as the yield is not reduced by nonuniform compression.
It is still another object of the invention to provide fuel pellets which, when imploded, will avoid instabilities between the imploding front and the fuel.
It is still another object of the invention to provide fuel pellets of maximum durability.
It is still another object of the invention to provide fuel pellets that will provide the most efficient distribution of energy yield when exposed to laser beams.
Other objects of the invention will impart the obvious and will in part appear hereinafter. The invention accordingly comprises several steps and the relation of one or more of said steps with respect to each of the other and the articles possessing the features, properties, and the relation of elements, which are exemplified in the articles and processes herein described. The scope of the invention is indicated in the claims.